Obesity represents the most prevalent of body weight disorders, and it is the most important nutritional disorder in the Western world, with estimates of its prevalence ranging from 30% to 50% of the middle-aged population. The number of overweight and obese Americans has continued to increase since 1960, a trend that is not slowing down. Today, 64.5 percent of adult Americans (about 127 million) are categorized as being overweight or obese. Each year, obesity causes at least 300,000 deaths in the U.S., and healthcare costs of American adults with obesity amount to approximately $100 billion (American Obesity Association).
Obesity increases an individual's risk of developing conditions such as high blood pressure, diabetes (type 2), hyperlipidemia, heart disease, hypertension, stroke, gallbladder disease, and cancer of the breast, prostate, and colon (see, e.g., Nishina, P. M. et al, Metab. 1994, 43, 554; Grundy, S. M. & Barnett, J. P., Dis. Mon. 1990, 36, 641). In the U.S., the incidence of being overweight or obese occurs at higher rates in racial/ethnic minority populations such as African American and Hispanic Americans, compared with Caucasian Americans. Women and persons of low socioeconomic status within minority populations appear to particularly be affected by excess weight and obesity. This trend is not limited to adults. Approximately 30.3 percent of children (ages 6 to 11) are overweight and 15.3 percent are obese. For adolescents (ages 12 to 19), 30.4 percent are overweight and 15.5 percent are obese. Diabetes, hypertension and other obesity-related chronic diseases that are prevalent among adults have now become more common in children and young adults. Poor dietary habits and inactivity are reported to contribute to the increase of obesity in youth. Additionally, risk factors for developing childhood obesity include having overweight parents, or parents unconcerned about their child's weight, increased energy intake due to larger serving sizes, increased sedentary lifestyle and decreased transport-related activity (walking to school or to the bus stop), having a temperament with high levels of anger/frustration (which may cause parents to give their child extra food and calories to decrease tantrums), having Down's Syndrome, mother's pregnancy body mass index (BMI) and first born status (increased prevalence of obesity). One tool used for diagnosing obesity in adults is calculating an individual's BMI, which is a measure of body weight for height (Garrow & Webster, International Journal of Obesity 1985, 9, 147). A BMI of 25 to 29.9 indicates that an individual is overweight, while a BMI of 30 or above is indicative of obesity. For children, BMI is gender and age specific (Pietrobelli et al, Journal of Pediatrics 1998, 132, 204). Risk factors for developing obesity in adulthood include poor diet (high-calorie, low nutrients); lack of physical activity; working varied shifts; quitting smoking, having certain medical conditions such as rare hereditary diseases, and hormonal imbalances (such as hypothyroid, Cushing's disease and polycystic ovarian syndrome); certain medications (steroids and some antidepressants); being a racial or ethnic minority (especially a female minority); low socioeconomic status; age (increased risk from 20-55), pregnancy; and retirement (due to altered schedule).
Melanocortin 4 Receptor and Obesity
Melanocortin (MC) receptors are members of the seven-transmembrane-domain G protein-coupled receptor superfamily that activate generation of the second messenger cyclic AMP (cAMP). There are five MC receptors isolated to date: MC1R, MC2R, MC3R, MC4R and MC5R. Human MC4R is 332 amino acids in length. The melanocortin 4 receptor (MC4R) has been implicated in the regulation of body weight (Graham et al, Nat. Genetics 1997, 17, 273). MC4R is expressed in the brain, including the hypothalamus, which influences food intake (Markison & Foster, Drug Discovery Today, 2006, 3, 569).
Signaling via MC4R stimulates anorexigenic neural pathways. MC4R null mice develop late onset obesity with hyperglycemia and hyperinsulinemia. Mice lacking one MC4R allele (heterozygotes) have intermediate body weight between wild-type and homozygous null mice. Transgenic mice overexpressing an endogenous MC4R antagonist, agouti-related protein (AgRP), exhibited increased weight gain, food consumption, and body length compared with non-transgenic littermates (Oilman et al., Science 1997, 278 135). In humans, MC4R deficiency is the most common monogenic form of obesity (Farooqi et al., New Engl. J. Med. 2003 348, 1085). Numerous mutations affecting MC4R activity have been found and many are associated with obesity including early-onset (childhood) obesity (Nijenhuis et al., J. Biol. Chem. 2003, 278, 22939; Branson et al., New Eng. J. Med. 2003, 348, 1096; Gu et al., Diabetes 1999, 48, 635; Tao et al., Endocrinology 2003, 144, 4544). Recently, pharmacological restoration of mutant melanocortin-4 receptor signaling with cell permeable MC4R ligands has also been reported (René et al. J. Pharmacol. Exp Ther. 2010, 335, 520).
Several authors have now reviewed the recent advances in our understanding of the genetics of MC4R in early onset obesity (e.g., Farooqi IS & O'Rahilly S, Int J Obes (Lond), 2005 Oct., 29(10), 1149; Govaerts et al., Peptides, 2005 Oct., 26(10), 1909; Tao Y X, Mol Cell Endocrinol, 2005 M 15, 239(1-2), 1-14; Farooqi IS & O'Rahilly S, Annu Rev Med, 2005, 56, 443-58). For example, in one patient with severe early-onset obesity, an autosomal-dominant mode of inheritance of an MC4R mutation has been found to be due to a dominant-negative effect caused by receptor dimerization (Biebermann H et al. Diabetes, 2003 December, 52(12), 2984).
Natural agonists (ligands) of MC4R include [alpha]-MSH, ACTH, [beta]-MSH, and [gamma]-MSH (in order from highest to lowest affinity). Other MC4R ligands, including agonists and antagonists, which have been described to date are peptides (U.S. Pat. No. 6,060,589) and cyclic peptide analogs (U.S. Pat. No. 6,613,874 to Mazur et al.). Further, U.S. Pat. Nos. 6,054,556 and 5,731,408 describe families of agonists and antagonists for MC4R that are lactam heptapeptides having a cyclic structure. A series of MC4R peptide agonists have also been designed (Sun et al., Bioorg Med Chem 2004, 12(10):2671). In addition, Nijenhuis et al. (Peptides 2003, 24(2):271) described the development and evaluation of melanocortin antagonist compounds that were selective for the MC4R. One compound, designated Ac-Nle-Gly-Lys-D-Phe-Arg-Trp-Gly-NH(2) (SEQ ID NO:9), was found to be the most selective MC4R compound, with a 90- and 110-fold selectivity for the MC4R as compared to the MC3R and MC5R, respectively. Subsequent modification yielded compound Ac-Nle-Gly-Lys-D-Nal(2)-Arg-Trp-Gly-NH(2) (SEQ ID NO: 10), a selective MC4R antagonist with 34-fold MC4R/MC3R and 109-fold MC4R/MC5R selectivity. Both compounds were active in vivo, and crossed the blood-brain barrier. On the other hand, it was recently shown that the moderately selective peptide antagonist PG-932 (7-fold MC4R/MC3R selectivity) increased food intake in mice upon peripheral administration (Sutton et al. Peptides, 2008, 29, 104). A recent report also describes the activation of mutated MC4R by novel peptide agonists (Roubert et al., J. of Endocrinology, 2010, 207, 177).
Other high-affinity MC4R antagonists are described in Grieco et al. (J Med Chem 2002; 24:5287). These cyclic antagonists were designed based on the known high affinity antagonist SHU9119 (Ac-Nle4-[Asp5-His6-DNal(2′)7-Arg8-Trp9-LyslO]-NH(2)) (SEQ ID NO: 11). The SHU9119 analogues were modified in position 6 (His) with non-conventional amino acids. One compound containing a Che substitution at position 6 is a high affinity MC4R antagonist (1050=0.48 nM) with 100-fold selectivity over MC3R. Another compound with a Cpe substitution at position 6 also was a high affinity MC4R antagonist (1050=0.51 nM) with a 200-fold selectivity over MC3R. Molecular modeling was used to examine the conformational properties of the cyclic peptides modified in position 6 with conformationally restricted amino acids. See also, Grieco et al., Peptides 2006, 27, 472. Several non-peptide MC4R ligands have also been disclosed in U.S. published patent applications 2003/0158209 to Dyck et al. and 2004/082590 to Briner et al. Also, U.S. Pat. No. 6,638,927 to Renhowe et al. describes small, low-molecular weight guanidobenzamides as specific MC4R agonists. Richardson et al. have described novel arylpiperizines that are agonists of MC4R (J Med Chem 2004; 47(3), 744). U.S. Pat. No. 6,979,691 to Yu et al. and U.S. Pat. No. 6,699,873 to Maguire also describe non-peptide compounds which bind selectively to MC4R. WO 99/55679 to Basu et al. discloses isoquinoline derivatives, small molecule non-peptide compounds, which show low (micromolar) affinities for the MC1R and MC4R, reduction of dermal inflammation induced by arachidonic acids, and reductions of body weight and food intake. WO 99/64002 to Nargund et al. also discloses spiropiperidine derivatives as melanocortin receptor agonists, useful for the treatment of diseases and disorders such as obesity, diabetes, and sexual dysfunction. A large number of MC4-receptor ligands developed recently are analogs of N-acylpiperidines or piperazines (Nozawa et al. Expert Opin. Ther. Patents, 2008, 18, 403).
Other non-peptide MC4R antagonists have been described. Thus, U.S. published patent applications 2003/0176425 and 2003/0162819 to Eisinger et al. disclose novel 1,2,4-thiadiazole and 1,2,4-thiadiazolium derivatives, respectively, as MC4R antagonists or agonists. These applications also disclose use of these compounds to treat obesity. Other MC4R binding compounds are described in the following: Singh et al. J. Med. Chem, 2011, 54, 1379; Mayorov et al., Bioorg. Med. Chem. Lett., 2011, 21, 3099; Conde-Frieboes et al., Bioorg. Med. Chem. Lett., 2011, 21, 1459; Hong et al., Bioorg. Med. Chem. Lett., 2011, 21, 3099; DeBoer, Nutrition, 2010, 26, 146. He et al. Bioorg. Med. Chem. Lett. 2010, 20, 6524. Emmerson et al., Curr. Top. Med. Chem. 2007, 7, 1121. Nargund et. al. J. Med. Chem. 2006, 49, 4035. Guo et al. Bioorg. Med. Chem. Lett. 2008, 18, 3242. Sebhat et al. Bioorg. Med. Chem. Lett. 2007, 17, 5720. Chen et al. Bioorg. Med. Chem. 2008, 16, 5606. Marinkovic et al. Bioorg. Med. Chem. Lett. 2008, 18, 4817. Tran et al. Bioorg. Med. Chem. Lett. 2008, 18, 1124. Tran et al. Bioorg. Med. Chem. Lett. 2008, 18, 1931. Bednarek & Fong, Exp Opn Ther Patents 2004, 14, 327; Ujjainwalla et al., Bioorg. Med. Chem. Lett. 2005, 15, 4023; WO11/054285 (Zhang Ga); WO 10/144344 (Palatin); WO 10/065801 (Palatin); WO 10/037081 (Palatin); WO 10/065802 (Palatin); WO 10/065801 (Palatin); WO 10/065800 (Palatin); WO 10/065799 (Palatin); WO 03/07949 (Merck); WO 10/081666 (Santhera); WO 10/034500 (Santhera); WO 09/080291 (Santhera); WO 09/115321 (Santhera); WO 04/075823 (Ipsen); WO 04/089951 (Ipsen); WO 05/056533 (Ipsen); WO 06/010811 (Ipsen); WO 03/61660 (Eli Lilly); WO 03/09847 (Amgen); WO 03/09850 (Amgen); WO 03/31410 (Neurocrine Biosciences); WO 03/94918 (Neurocrine Biosciences); WO 03/68738 (Neurocrine Biosciences); WO 03/92690 (Procter and Gamble); WO 03/93234 (Procter and Gamble); WO 03/72056 (Chiron); WO 03/66597 (Chiron); WO 03/66587 (Chiron); WO 03/66587 (Chiron); WO 02/67869 (Merck); WO 02/68387 (Merck); WO 02/00259 (Taisho); WO 02/92566 (Taisho); WO 02/070511 (Bristol-Myers Squibb); WO 02/079146 (Bristol-Myers Squibb); WO 10/056022 (LG Life Sciences); Pontillo et al., Bioorg Med Chem Lett. 2005, 15, 5237; Pontillo et al., Bioorg Med Chem Lett. 2005, 15, 2541; Pontillo et al., Bioorg Med Chem Lett. 2004, 14, 5605; Cheung et al., Bioorg Med Chem Lett. 2005, 15, 5504; Yan et al., Bioorg Med Chem Lett. 2004 15, 4611; Hsiung et al., Endocrinology. 2005, 146, 5257; and Todorovic et al., Peptides. 2005 Oct., 26(10), 2026.
Current Treatments
Current anti-obesity drugs have limited efficacy (Jones, Nat. Rev. Drug Discov. 2009, 8, 834. Yao & Mackenzie, Pharmaceuticals, 2010, 3, 3494) and numerous side effects (Crowley et al., Nat. Rev. Drug Discov. 2002; 1, 276). With obesity reaching epidemic proportions worldwide, there is a pressing need for the development of adequate therapeutics in this area. In recent years, hormones and neuropeptides involved in the regulation of appetite, body energy expenditure, and fat mass accumulation have emerged as potential anti-obesity drugs (McMinn, et al., Obes Rev 2000, 1, 37; Drazen, D. L. & Woods, S. C, Curr Opin Clin Nutr Metab Care 2003, 6, 621). At present, however, these peptides require parenteral administration. The prospect of daily injections to control obesity for extended periods of time (since obesity is a chronic condition) is not very encouraging and limits the use of these drugs.
Thus, there is a need for improved pharmacological agents that are useful to treat obesity in humans.